The woman, believed to be in her 20s, was pronounced dead at the scene after building work fell on her in Hanover Square in London's expensive Mayfair, the London Ambulance Service (LAS) said.

An air ambulance was sent and extensive efforts were made to resuscitate the woman to no avail. An LAS spokeswoman said: "We were called at 11.30am today to reports of building work fallen on a person at Hanover Square. Extensive efforts were made to resuscitate a woman, reported to be in her 20s, but sadly she was pronounced dead at the scene by the air ambulance doctor."

Police also attended the scene. A Scotland Yard spokeswoman said: "At this stage it is believed an unknown object hit the woman."

A Health and Safety Executive (HSE) spokeswoman said they were aware of the incident and were "making initial inquiries".

A yellow and white police forensics tent that a witness said contained the woman's body was in place outside 5 Hanover Square. The south-east corner of the square was cordoned off by police and people in blue overalls and hard hats could be seen taking photos and inspecting the area.

The witness, who did not wish to be named, said workers from the building opposite had been sent home. The building houses a fine art gallery on the ground floor, according to its website.

A woman who works in the Itsu food store opposite the building said she saw a young woman lying on the ground with two paramedics attempting to resuscitate her by pumping her chest.

She said: "I just saw her quickly then I closed my eyes, it was not very nice. I could see her legs and them (ambulance staff) pumping her chest. I think it was a young girl but I didn't want to look any more."

Three workers on a cigarette break outside Vogue House, which also overlooks 5 Hanover Square, said barricades were in place around the building but were removed a few months ago. One woman said: "We walk past there every day. It's quite scary that it could have been any of us."